1. Field of the Invention
The present invention relates to cellular radio telecommunication systems and, more particularly, to a method and system for uniquely locating the source of a radio signal within the cells of a cellular radio system.
2. Description of the Related Art
In a cellular radio telecommunications system, the area within which a plurality of mobile stations are served by the system is divided into cells. Each idle and active mobile station within a cell remains in radio contact with the base station serving that cell. In conventional operation, when a mobile station that is engaged in active communication moves from one cell to another cell the cellular system performs a hand-off in which the moving mobile station is instructed to retune its radio to a new traffic channel which is served by the base station of the cell it is now entering.
A cellular system must know the approximate location of each of the mobile stations engaged in active communication in order to provide efficient hand-off and traffic management functionality. Location information is generally provided by the measurement of signal strength of radio signals passing between the mobile station and its serving base station as well as radio signals of neighboring base stations serving the cells in the surrounding geographic area.
Cellular radio telecommunication systems require information concerning a mobile station's location primarily for hand-off administration and other traffic management functions. As a mobile station traverses the cell boundaries of a system it is handed off from one cell to another. In North American analog cellular systems, once a mobile station's transmitted signal is perceived as being below a preselected threshold value at the base station currently serving that mobile, the serving base station sends a measurement request to neighboring cells asking for a measurement of the received signal strength indication (RSSI) of the mobile station's signal by the base stations serving these neighboring cells. If the mobile's current serving base station does not have the strongest RSSI the system will identify the cell with the strongest RSSI as the target cell for a hand-off; that is, the next cell to serve the mobile station when hand-off is executed.
In the North American dual mode digital cellular system, implemented in accordance with IS-54 and IS-136, digital mobile stations are equipped with mobile assisted hand-off (MAHO) capabilities. When MAHO is activated, a mobile station periodically measures and reports the strength of the signal transmitted from its serving base station as well as the strengths of signals from up to 12 other neighboring base stations. The network of the serving base station may use MAHO-reported signal strength in the downlink (base to mobile) direction together with the signal strength in the uplink (mobile to base) direction, together with the decoded color code, measured by neighboring base stations to make its hand-off decisions.
The Global System for Mobile (GSM) digital cellular radio system in use in Europe, North America and other parts of the world also provides a mobile system management function which enables a mobile station to identify neighboring base stations by means of signal measurements. A mobile is intended to be able to identify the source of the signal being received from one of its neighboring base stations by decoding the base station identity code (BSIC) which is transmitted on the broadcast control channel (BCCH) frequency of each of the cells. In the North American analog system (AMPS) and dual mode system (D-AMPS) a "color code" signal is instead transmitted by each of the base stations so that a mobile station can listen to the color code signal of its base station and then rebroadcast that color code to allow identification of the base station by which it is currently being served. In certain cases, the synchronization code sent in each burst can also be used as an identification of the origin of the signal.
Since the identity information encoded into the BSIC signal and in the color codes incorporated into the AMPS and D-AMPS signals consists of only a few bits and are reused within the same system, it is likely that more than one transmitter within a single large system may transmit the same identity information on the same frequency. Thus, it is impossible with these techniques for the network control of the system to uniquely identify a signal as having its source as being from within a particular cell or from a particular mobile on a system-wide basis.
When a mobile station is engaged in the call setup process with the cellular system it periodically broadcasts an identification of its identity, for example, its international mobile subscriber identity (IMSI). However, while the mobile is actively engaged in a call, it generally does not sent out any indicia which specifically identifies it and from which the system can associate that mobile with a specific traffic channel frequency being served by a specific base station within the system. Thus, the network cannot discern from the signal broadcast by the mobile on its traffic channel the specific identity of that mobile.
As cellular systems become more complex and network control systems become more sophisticated it is desirable to automate many of the different administrative functions which are conventionally done manually or with empirically derived parameters. For example, each cellular system must have a good frequency plan specifying which frequencies are reused in different cells of the system to maximize the availability of traffic channels to its subscribers throughout the system while minimizing cochannel interference between different cells. Most frequency plans are formulated today by either predicting interference between frequencies reused in different cells with a propagation model or by systematic measurement of system performance to develop empirical data and experience. For operators whose systems are growing rapidly with the frequent addition and rearrangement of base stations, these techniques of frequency planning are both slow and costly. Moreover, the recent introduction of microcells and picocells into a system make frequency planning even more difficult. If algorithms could be developed to actually measure and collect data on cochannel interference in each of the cells throughout a cellular system it would be a great advantage. For example, such measured and collected data would be more accurate than predicted values and would eliminate the need for digital maps and sophisticated propagation models and the associated time-consuming computations used in conventional frequency planning. The actual frequency allocation might be done either manually or the system itself could reallocate frequencies from time to time and, in effect, automatically amend the frequency plan for maximum efficiency of reuse. A principal drawback to the development of such adaptive algorithms is, however, the lack of sufficiently detailed system information to uniquely determine from which base station or mobile station a particular interfering signal is originating. Without a truly unique identity code associated with each traffic signal measured by a mobile station or base station it is difficult to identify the mobile or cell from which the interference is originating and develop an algorithm for identifying and correcting the interfering condition based upon measurements within the system.
Another exemplary administrative function within a cellular system which is a possible candidate for automation is the assembly of a neighbor cell list to be sent to a mobile during a call. This list specifies which neighboring cell carrier frequencies on which the mobile is to periodically measure the signal strength and report the results to the serving base station and the network for possible handoff purposes. Today the preparation of such lists is a non-trivial exercise of examining the coverage areas of different cells and deciding which are likely to be the best possible neighboring cells in each instance. If the system could directly measure and monitor the signal strength of the frequencies in various cells, and determine, for example, which frequencies in which cells are experiencing interference and from which other cells that interference is coming, such information could be used in an algorithm to prepare a best neighbor cell list. Similarly, a measured good frequency could be identified as being associated with the unique identity of the base station of its origin and added to the neighbor cell list. However, the BSIC value transmitted as part of the BCCH carrier frequency signal associated with each base station is not "system unique", and consequently insufficient for providing the basis for such adaptive algorithms.
For purposes within a cellular radio system such as that illustrated above, it is very important to identify the unique origin of a signal in order to increase the accuracy and ease with which frequency information is collected as well as to be able to develop algorithms for use within the system. The design of a self-configuring cellular system which adapts to changes in the infrastructure, due, for example to the addition or relocation of base stations within the system, would require the use of adaptive algorithms. Similarly, adaptive neighboring cell lists are necessary in order to design a self-configuring cellular system and unique identification of the participants involved in a call in another cell is necessary in order to develop smart adaptive cell list algorithms.